Glaucoma is a group of diseases of the eye characterized by progressive damage of the optic nerve that may result in visual field loss and eventual irreversible blindness if inadequately controlled. The World Health Organization currently ranks glaucoma as the second most common cause of blindness. As majority of patients do not experience any symptoms until the disease reaches an advanced stage, early detection and treatment is of utmost importance.
The diagnosis of glaucoma is based on the appearance of the optic nerve head (ONH) and visual field, as shown in FIG. 1. Accordingly, there has been increasing interest in the development of imaging technology that can detect the structural changes at the level of the optic disc and in the retinal nerve fiber layer. Quantitative measurement of the cupping of the ONH is generally used to evaluate the progression of glaucoma. Certain features of the optic disc have been identified to correlate with such progression, such as rim area, cup area, and C/D area ratio (see FIG. 2).
The conventional ophthalmic approaches to measuring three-dimensional (3D) structure and/or assessing ONH are Heidelberg Retina Tomography (HRT), stereo fundus camera imaging, and Optical Coherence Tomography (OCT), as shown in FIGS. 3(a)-3(c). HRT enables some of disc features to be objectively quantified, but it requires a manual delineation of the disc margin. This subjective definition leads to substantial inter-observer variability. Alternatively, disc photography is a common and cheap ophthalmic device widely used in fundus imaging for analysis and diagnosis. The assessment of the photographs is subjective and heavily dependent on the observer's skills, for instance, with using a stereo viewer (see FIG. 4).
Some studies have suggested automated methods to extract the disc parameters for analysis, but none of these methods has been commercialized. Moreover, the existing stereo disc photography could not get high resolution in depth due to the optical properties of cameras. And most importantly, depth information derived from stereo disc photos is mathematically calculated speculation rather than direct measurement.
Spectral domain optical coherence tomography (SD-OCT) is a new high resolution imaging technique, capable of achieving micrometer resolution in depth. It allows detailed imaging of eye structures (see, e.g., FIG. 5). The introduction of 3D OCT, which can provide 3D image (cube image) of the ONH, offers a promising means for automated analysis, early detection and monitoring the progression of glaucoma and other optic nerve head diseases. There is no automated system to date to extract the ONH information with SD-OCT.
The present inventors developed an automated ONH detection/analysis based on conventional stereo optic disc photos, and they proved its usefulness and efficiency previously (see citation [3], infra). As described above, SD-OCT has a potential to replace conventional technique and to provide more detailed, 3D structural information about the ONH, relying on direct measurement and not on a mathematical theory. The aforementioned detection/analysis cannot be implemented for stereo disc photos, because the nature of the information captured by SD-OCT is quite different from that of stereo disc photos.